My 300 SEL has a different problem: not enough heat. My Tech said is was NOT the monovalve. My dealer said it has something to do with the electric push button (around $300).

My heat only works when it is on LOW. When you put the heat on HI, it blows on cold air.

What is the problem and why so expensive to fix?

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)